Prevention of coke formation in steam cracking processes



United States Patent O 3,546,316 PREVENTION OF COKE FORMATION IN STEAMCRACKING PROCESSES Ihor Koszman, Parsippany, N.J., assignor to EssoResearch and Engineering Company, a corporation of Delaware No Drawing.Filed Feb. 9, 1968, Ser. No. 704,256 Int. Cl. Cg 1/16; C07c 3/08 U.S.Cl. 260-683 14 Claims ABSTRACT OF THE DISCLOSURE FIELD OF INVENTION Thisinvention relates to an improvement in the thermal cracking of arseniccontaining petroleum fractions. More particularly, this inventionrelates to an improved steam cracking process wherein coke formation inthe cracking zones is greatly inhibited by the removal of substantiallyall of the arsenic from an arsenic containing petroleum a feed fractionprior to the cracking operation.

PRIOR ART Thermal cracking processes, conducted with or without thepresence of steam, are well known to the art and are a major source ofhighly valuable unsaturated compounds, e.g., ethylene, butadiene, etc.Unfortunately, however, all of the petroleum feed fraction is notpyrolytically converted to desirable products. Thus, undesirable sidereactions also occur, which give rise to acute difficulties and tend tosubstantially reduce the yield of desirable products from the crackingprocess. One of the most troublesome side reactions occurring in suchprocesses is the formation of coke within the cracking zones, i.e.,within the tubes in the cracking furnace, the reaction mixture beingheated to cracking temperatures within these tubes. Generally, forexample, when ethane is cracked, a portion thereof is converted tomethylene radicals, i.e., CH radicals, and hydrogen. These radicals mayeither form ethylene, or by linking together, it is believed, to formpolymer. The polymer then dehydrogenates under the conditions existingin the tubes and forms carbon deposits. Certain forms of this carbon,i.e., coke, are in an extremely hard particulate form. The coke adheresto the tube walls and tends to build up, thereby restricting theeffective cross-sectional area of the tube and leading to large pressuredrops. Additionally, because of the insulating properties of the coke,furnace temperatures must be increased in order to maintain desiredreaction temperatures within the tubes. This reduces tube lifesubstantially and increases the frequency of shutdown periods forreplacing damaged tubes.

While coking is a normal feature of almost all steam cracking processes,it appears that the coking rate is substantially increased by thepresence of certain metals, for example, arsenic. Thus, since arsenic isknown to be present in various oil fractions (in several and changeableforms, e.g., arsine, arsenious oxide, etc., but hereinafter simplyreferred to as arsenic regardless of the form it takes), the cokingrate, when such fractions are steam cracked, is substantially increased,a harder coke forms, and coke removal operations are, accordingly, morefreice quent and more difficult. For example, onstream periods maygenerally range from about 1000-1400 hours but due to arsenic promotedcoking, onstream periods have been reduced to less than about 500 hours,and as low as 350 hours. It has now been discovered, however, thatcoking can be substantially inhibited, and the product yield from thesteam cracking of arsenic containing hydrocarbons can be greatlyincreased, i.e., by allowing greater run periods, by the substantialremoval of arsenic from arsenic containing feeds.

SUMMARY OF THE INVENTION In accordance with this invention, therefore,an improved steam cracking process is provided wherein the formation ofcoke, in the steam cracking of an arsenic containing petroleum feed, issubstantially reduced by treating the feed to reduce substantially thearsenic concentration therein. While not wishing to be bound by anyparticular theory, it is believed that arsenic, in whatever form it ispresent in the petroleum feed, tends to promote the formation of coke,leading to shorter run times and greater downtimes. Additionally, it hasbeen found that normal decoking operations, e.g., air decoking, fortubes through which arsenic containing feeds have been cracked do notremove all of the arsenic containing coke. Consequently, since arsenictends to concentrate in the coke, sufiicient arsenic remains to promoteexcessive coke formation in the subsequent cracking of feeds containinglittle or no arsenic. Therefore, the necessity for removing arsenic fromthe feed prior to steam cracking is clearly evidenced.

Arsenic removal can be effected in a variety of ways, well known to theart. Generally, any procedure adapted for the removal of arsenic frompetroleum fractions will suffice to reduce the arsenic concentration ofthat fraction to levels insufficient to cause excessive coking when thatfraction is thermally cracked. One skilled in the art will encounterlittle trouble in determining the optimum arsenic removal technique andoptimum arsenic concentrations for the most efiicient results. Thus,once knowing that it is arsenic that causes excessive coking, thedetermination of the arsenic removal technique to reach satisfactoryarsenic levels will be a relatively straightforward procedure.Preferably, however, the arsenic removal process is conducted in such amanner as to reduce the arsenic concentration of the steam cracking feedto less than about 50 p.p.b. (part per billion) by weight, morepreferably, less than about 10 p.p.b., and still more preferably lessthan about 2 p.p.b.

Many arsenic removal techniques have been reported in the literature anda few will be discussed hereinbelow. For example, one method forremoving arsenic from petroleum fractions involves the use of a basiccompound, i.e., when dissolved in water the pH is above about 7, ofalkaline or alkali earth metals, e.g., oxides, hydroxides, salts, suchas the hydroxides of lithium, sodium, potassium rubidium, cesium,barium, calcium, strontium, etc., sodium salts such as acetate,phosphate, borate, carbonate, citrate, cyanide, chromate, formate,lactate, oxalate, perborate, tartrate, etc., and similar salts of theother alkaline and alkali earth metals. These compounds may be employedin aqueous solutions or solutions of polar solvents such as alcohols,ethers, ketones, etc., or used as a solid bed with no solvent, or usedas a supported material on such suitable carrying materials askieselguhr, alumina, silicates, magnesia, zirconia, titania, etc. Thearsenic containing petroleum fraction, in the liquid or gaseous phasedepending upon contact temperatures and feed to be treated, is contactedwith the basic reagent at temperatures below about 500 F., preferablyabout -500 F. A fraction of reduced arsenic content is then recovered bynormal procedures. More details on this method can be found in US.2,779,715.

Another method for removing arsenic from petroleum fractions is reportedin U.S. 2,781,297. This procedure involves contacting the arseniccontaining petroleum fraction with a salt of copper and/ or metals lowerthan copper in the electromotive series, e.g., mercury, silver,palladium, platinum, gold, etc. Generally, the salts are those of acids,such as sulfates, chlorides, nitrates, fluorides, etc., but may also beorganic acid salts such as acetates, propionates, butyrates, valerates,etc. The salt is preferably composited with a porous support, such askieselguhr, silica gel, alumina, magnesia, silica, clays, and the like,and contact with the petroleum fraction may be had at any suitabletemperature, e.g., but generally below about 500 F. The condition of thefeed will again depend upon contact temperatures and the feed to betreated.

A third method for reducing the arsenic concentration of petroleumfractions involves the use of a nitrogen compound containing threeattached groups and an unshared pair of electrons, e.g., aqueousammonia, hydrazine, alkanol amines, aliphatic amines, alkylenepolyamines, and the like, some typical examples of which are: ethylamine, dimethyl formamide, formamide, ethanolamine, diethylene triamine,acetamide, tetraethylene penta-amine, pentaethylene hexa-amine, ethylenediamine, hexanol amine, etc. The nitrogen compounds may be used as suchor in solutions of water, alcohols, ketones, etc. The petroleum fractionis contacted with the reagent at temperatures generally not above about200 F. and an arsenic free petroleum fraction is recovered. In U.S.2,867,577 further details on this process are reported.

Another method for removing arsenic from petroleum fractions involvesthe use of an acid impregnated support, the acid being present inamounts above about 20 wt. percent, preferably about 50-150 wt. percent,e.g., sulfuric acid impregnated silica gel as reported in US. 3,093,574.The petroleum fraction is contacted with the acid impregnated support attemperatures below about 500 F., but preferably below about 200 F and apetroleum fraction substantially reduced in arsenic content is thenrecovered. In addition to silica gel, other porous support materials maybe acid impregnated, e.g., kieselguhr, clays, diatomaceous earths, etc.,and similarly other acids may be employed, e.g., phosphoric acids,nitric, and the like, but preferably those acids that contain oxygen.(Various US. patents have been mentioned hereinabove as disclosingtechniques for the removal of arsenic from petroleum fractions. Thedisclosure of these patents regarding treatment of the petroleumfraction for arsenic removal is hereby incorporated herein byreference.)

Still another, and preferred, method for removing arsenic from petroleumfeeds involves the use of activated charcoal. The employment ofactivated charcoal or any other similar porous material, e.g., alumina,molecular sieves, silica-alumina, cobalt-silica-alumina, clays, e.g.,atapulgite clays, diatomaceous earths, etc., is in a manner similar tothat for employing silica gel, i.e., operating conditions areessentially the same. Additionally, the activated charcoal may beimpregnated with acids such as sulfuric acid, or with metals such ascopper, which tend to increase the arsenic removal process by allowingfor a reaction, i.e., conversion to arsenates which are a readilyremovable form, in addition to the physical adsorption process alreadyoccurring.

The foregoing processes can be conducted with the petroleum feedfraction either in the liquid or vapor phase depending upon the state ofthe feed fraction at the particular temperature at. which the arsenicremoval step is effected. Additionally, the feed may be processed one ormore times through the arsenic removal medium in order to reach desiredarsenic levels. The arsenic free petroleum fraction may be recovered byany well-known technique,

such as filtration, centrifugation, etc.

The foregoing arsenic removal processes are normally satisfactory forremoving sufficient arsenic from the petro. leum feed to preventexcessive coking in the cracking operation. Thus, while the arsenicconcentration in petroleum feed fractions generally ranges from about200-800 p.p.b. and higher, the foregoing processes should be employledto reduce this concentration to previously specified eve s.

The cracking operation is well known and will be only briefly discussedhere. See, for example, Chemical Week, Nov. 13, 1965, page 72 et seq.Generally, the petroleum feed fraction is admixed with steam, i.e., inamounts ranging from about 20 to mol percent steam, preferably about 20to 60 mol percent, prior to entry into the steam cracking furnace. Thefurnace normally contains two sections, a convection section wherein thefeed is vaporized, if not already in that form, and a radiant orcracking section, the feed being passed in admixture with steam in theabsence of added catalysts through one or more tubes located within thefurnace. The convection section is normally employed to increase heatingefficiency and the petroleum-steam mixture is heated therein totemperatures of about 1000 to 1100 F How ever, these temperatures arebelow that at which the feed cracks since cracking is undesirable in theconvection section. The heated feed then passes into the radiant sectionwhere the temperature is quickly raised to about 1200 to 1700 F, orhigher as tube metal materials permit, and the feed is cracked.Residence times in the radiant section are carefully controlled tominimize coke formation, polymerization, and other undesirablereactions. Thus, residence times in the cracking section will range from0.1 to 10 seconds, preferably about 0.1 to 1 second. Pressures withinthe tubes may range from about 0-50 p.s.i.g. but are not critical, andhigher pressures, e.g., up to about 100 p.s.i.g., can be tolerated. Uponexiting the cracking section, the reaction products are immediatelyquenched to stop further reactions and/or minimize loss of primaryconversion products.

The petroleum fractions which may be converted by this process may varywidely in boiling point range. Generally, however, the process is mostapplicable to hydrocarbon feeds consisting essentially of cyclic oracyclic saturated hydrocarbons. Thus, hydrocarbon feeds that may beutilized herein include such cyclic hydrocarbons as cyclopropane,cyclopentane, cyclooctane, and mixtures thereof. The acyclic hydrocarbonfeeds include any alkane, namely, aliphatic hydrocarbons of the methaneseries or mixtures of alkanes with eycloalkanes. The preferred feeds foruse are saturated hydrocarbons containing from 2 to 24 carbon atoms and,more preferably, alkanes containing from 2 to about 12 carbon atoms.Exemplary hydrocarbon feeds which can be used in the practice of thisinvention are butane, ethane, propane, isobutane, n-hexane, n-decane,n-dodecane, n-hexadecane, eiscosane, tricosane and light naphthasboiling, at standard pressures, within a range of from about to 430 F.In addition to the foregoing, gas oils having boiling points ranginggenerally from about 450 to about 800 F. and kerosenes having a boilingtemperature ranging from about 430 to about 550 F. can also be utilizedin the practice of this invention.

The determination of the cause of coke formation is often a difficulttask but one method is to determine the concentration of variouselements in the coke produced by a cracking operation. Since traceelements tend to concentrate in the coke, they may be more easilydetected therein. Thus, it was found that the arsenic content of cokefrom a cracking operation where onstream periods had been reduced wasabout 20 ppm. to about 0.14% by weight and generally about 200-500p.p.m., while the arsenic content of coke from a normally runningprocess was only about 0-10 p.p.m., with values in the lower end of therange predominating.

In order to determine for certain that arsenic was indeed the cause ofobserved excessive coking rates several experimental tests wereconducted. These tests were conducted in a 3 inch long by 1 inchdiameter cracking tube of 310 stainless steel. A test feed of ethane andsteam were separately preheated to about l000 F., mixed, and passedthrough the cracking tube which was maintained at temperatures of1500l600 F. by electrical heating, thereby simulating a steam crackingfurnace. The steam content of the mixture was 25 weight percent and flowrates were adjusted to achieve conversions matching commercial units,thereby simulating cracking chemistry.

In a series of tests, gram coke samples, in which sample B containedarsenic concentration levels in the range of 20500 p.p.m. and sample Acontained arsenic concentration levels of an imperceptible amount, wereplaced in the cracking tube and a two hour ethane cracking test wasconducted at constant sulfur levels of p.p.m. for all tests. Table I,below, shows the weight percent change of the coke samples after threesuch tests.

The results of Table I show that relatively high arsenic levels in thecoke promote excessive coke formation, and the rate of coke formation ofcoke with high arsenic levels is at least 20 times that of coke with lowarsenic levels.

iIIl another series of tests about 1 gram of arsenious acid wassprinkled inside a new cracking tube and a sulfur free ethane-steammixture was cracked (this mixture, in the absence of arsenic, was foundnot to coke on fresh tube walls). After 2 hours 17.9 grams of coke hadbeen formed. The tube was then air decoked (not all of the coke beingremoved to simulate commercial practices) and another 2 hour testconducted. 11.7 grams of coke formed in this second test. The test wasrepeated seven times, with air decokin-g after each test, before thetube reached a noncoking level. These tests demonstrate the greattendency of arsenic to adhere to tube walls and to continue to promotecoking.

In another series of tests 500 ppb. arsine (AsH was added to the ethanetest feed. The coking rate upon cracking the test feed was 23 :g./ 2hrs., which is equivalent to a high coking rate in normal commercialsteam crackers, e.g., when the feed contains excessive amounts ofsulfur.

It follows from the foregoing test procedure that arsenic causesexcessive coking and its removal is highly desirable. While ethane wasused in these reported tests, since it is generally representative ofthe steam cracking feeds, similar results are obtained with otherpetroleum feeds.

What is claimed is:

1. In a process for thermally cracking an arsenic containing petroleumfeed stock by passing the same in admixture with steam in the absence ofadded catalysts through one or more tubes of a cracking furnace, theimprovement which comprises treating the arsenic containing feed stockso as to substantially reduce the arsenic content thereof therebysubstantially reducing the formation of coke on the interior of saidtube or tubes.

2. The process of claim 1 wherein the arsenic content of the feed isreduced after treating to less than about 50 p.p.b.

3. The process of claim 1 wherein the arsenic content of the feed isreduced after treating to less than about 10 p.p.b.

4. The process of claim 1 wherein the arsenic content of the feed isreduced after treating to less than about 2 p.p.b.

5. The process of claim 1 wherein arsenic removal is efiected by aprocess which comprises contacting the petroleum feed with a poroussupport under conditions effective for removing at least a substantialportion of the arsenic from the petroleum feed.

6. The process of claim 5 wherein the support is activated charcoal.

7. The process of claim 5 wherein the support is silica gel and isacidified with at least about 20 wt. percent of sulfuric acid.

8. The process of claim 5 wherein the support is kieselguhr and isacidified with at least about 20 wt. percent phosphoric acid.

9. The process of claim 1 wherein arsenic removal is effected by aprocess which comprises contacting the petroleum feed with an alkalinematerial selected from the group consisting of alkali metal and alkalineearth metal compounds, which when dissolved in water, have a pH aboveabout 7.

10. The process of claim 1 wherein arsenic removal is elfected by aprocess which comprises contacting the petroleum fraction with nitrogencompound containing three attached groups and an unshared pair ofelectrons.

11. The process of claim 1 wherein arsenic removal is effected by aprocess which comprises contacting the petroleum fraction with a salt ofa metal not higher than copper in the electromotive series of metals.

12. In a process for the production of olefinic products comprisingpassing an arsenic containing petroleum feed stock in admixture with 20to mole percent steam in the absence of added catalysts through one ormore tubes of a cracking furnace at a temperature between about 1200 and1700 F., the improvement which comprises treating said arseniccontaining petroleum feed stock prior to passage to said furnace toreduce the arsenic concentration thereof to less than about 50 parts perbillion by weight thereby substantially reducing the formation of cokeon the interior of said tube or tubes.

13. The process of claim 12 wherein said arsenic containing petroleumfeed stock is selected from the group consisting of saturatedhydrocarbons containing from 2 to 24 carbon atoms, light naphthasboiling within a range of from about 90 to 430 F., gas oil having aboiling point ranging from between about 450 to 800 F. and keroseneboiling between about 430 to about 550 F.

14. The process of claim 12 wherein said petroleum feed stock is ethane.

References Cited UNITED STATES PATENTS 2,203,470 6/1940 Pier et a1. 2082,865,838 12/1958 Mills 208- 2,867,577 l/1959 Urban et al. 208--2892,951,804 9/1960 Juliard 208-91 3,093,574 6/1963 Bertolacini et al.208-91 DELBERT E. GANTZ, Primary Examiner C. E. SPRESSER, IR., AssistantExaminer US. Cl. X.R.

